1. Field of the Invention
The present invention relates to a heat exchanger and more particularly to a heat exchanger and method for making employing boron nitride powder between the heat exchange tubing and the body of the exchanger.
2. Description of the Prior Art
In the prior art a stainless steel tubing has been employed embedded in an aluminum casting such as aluminum alloy 356-T5 and machined to a final form. The disadvantages of this is that the castings are porous and it is difficult to predict as to where porosity will occur. The casting alloys are less thermally conductive than for instance aluminum alloy 6061T6. Stainless steel tubing is less thermally conductive than copper tubing and the yield from such a process is quite low resulting in a high cost.
U.S. Pat. No. 4,852,645 Coulon, et al. discloses a heat transfer device for use between two materials which have different expansion coefficients. The thermal transfer layer comprises expanded graphite, for example, inserted between the materials which are selected from among carbonaceous materials, ceramics and metal or metal alloys. The expanded graphite is either inserted in the form of a rolled or compressed sheet or is compressed in place. Coulon forms the structure by forming at least one semicircular passage in each element. He discloses the use of silicon nitride which only has the thermal conductivity of 19 BTUs.times.ft/(hr.times.ft.sup.3 .times..degree. F.) to 105 BTUs.times.ft/(hr.times.ft.sup.3 .times..degree. F.) for boron nitride and his device is for use in applications such as chemical reactors, combustion devices or continuous casting of molten metals.
U.S. Pat. No. 4,024,620 Torcomian discloses the use, such as in FIG. 7, of filling a space between conductors 100 and 102 and 94 with metal wool fibers such as 98. Also FIG. 4 discloses the use of putting the two cooling coils 72 and 74 into the structure 66 and clamping them with walls 76 and 78.
U.S. Pat. No. 4,217,954 Vincent and U.S. Pat. No. 4,162,061 Buehler, et al. disclose a plurality of different layers between a cooling tube and the body.
A method of constructing a heat exchanger out of three pieces with an embedded tube surrounded by a conductive material is partially disclosed in Smith, U.S. Pat. No. 1,982,075 which shows a tube 27 in FIGS. 6 and 7 enclosed between 21 and 24 and conformed to fit but does not disclose any material therebetween such as boron nitride. Wittel, U.S. Pat. No. 4,583,583, discloses a similar structure using a teflon tube.
It is accordingly an object of the present invention to provide an improved heat exchanger which may employ an aluminum alloy other than a casting.
Another object of the invention is to provide such a heat exchanger containing boron nitride between the heat exchange tubing and the body.
A still further object of the present invention is to provide a method for forming such a heat exchanger.